1. Field of the Invention
This invention relates to crystal oscillation circuits using crystal oscillators (or quartz oscillators) and metal-oxide semiconductor transistors.
2. Description of the Related Art
FIG. 4 shows a typical example of a crystal oscillation circuit, which comprises inverters INV1 and INV2, a resistor (or resistance) Rf, a crystal oscillator X, and capacitors (or capacitance) Cg and Cd. Herein, the inverters INV1 and INV2 are each formed by CMOS (Complementary Metal-Oxide Semiconductor) transistors formed on an IC (Integrated Circuit) chip. The resistor Rf is externally arranged and is connected between terminals XIN and XOUT of the IC chip. The inverter INV1 acts as an inverting amplifier for exciting the crystal oscillator X, wherein the input thereof is connected to the terminal XIN, and the output is connected to the terminal XOUT. The inverter INV2 shapes the waveform of the output signal of the inverter INV1, wherein the input of the inverter INV2 is connected to the output of the inverter INV1.
Specifically, the inverter INV1 comprises a p-channel metal-oxide semiconductor field-effect transistor (hereinafter, referred as a PMOS transistor) P1 and an n-channel metal-oxide semiconductor field-effect transistor (hereinafter, referred to as a NMOS transistor) N1, which are connected in series between a voltage supply Vdd and the ground. These transistors P1 and N1 are coupled together in such a way that the gates thereof are both connected to the terminal XIN, and the drains thereof are both connected to the terminal XOUT. Similarly, the inverter INV2 comprises a pair of a PMOS transistor P2 and a NMOS transistor N2, which are connected in series between the voltage supply Vdd and the ground. This inverter INV2 inverts an output signal of the inverter INV1 to provide an oscillation output OSC.
The resistor Rf is arranged to set the inverter INV1 in such a way that the operating point is placed in a specific operational range providing a relatively large gain. That is, the resistor Rf is connected between the input and output of the inverter INV1 via the terminals XIN and XOUT. The crystal oscillator X is used to regulate the frequency of the oscillation output OSC (i.e., oscillation frequency). This crystal oscillator X is also connected between the input and output of the inverter INV1 via the terminals XIN and XOUT. These terminals XIN and XOUT are grounded via the capacitance Cg and Cd respectively.
The crystal oscillation circuit of FIG. 4 causes oscillation by way of a feedback loop that is formed by the inverter INV1 and the crystal oscillator X. When the oscillation frequency is shifted from the characteristic frequency of the crystal oscillator X, the crystal oscillator X may demonstrate inductive or capacitive performance in response to the frequency shifting direction, so that the oscillation frequency may be stabilized in proximity to the characteristic frequency. Thus, it is possible to produce the oscillation output OSC having the oscillation frequency that is regulated by the characteristic frequency of the crystal oscillator X. In order to demonstrate the oscillation of the crystal oscillation circuit described above, the gain of the inverter INV1 at a small-amplitude mode should be sufficiently increased to be greater than the loss of the crystal oscillator X. For this reason, the gain of the inverter INV1 is set to be sufficiently high.
Recently, electronic devices are developed in highly integrated manners and are reduced in size. Therefore, crystal oscillators should be correspondingly reduced in dimensions. As crystal oscillators are reduced in sizes, more severe restrictions should be required for excitation levels supplied to crystal oscillators. When the gain of the inverter for exciting the crystal oscillator is being increased so much in order to satisfy prescribed oscillation conditions, excitation levels should exceed prescribed limits of the crystal oscillator, which may cause unwanted damage to the crystal oscillator.
It is an object of the invention to provide a crystal oscillation circuit that would not cause damage to a crystal oscillator even though a relatively high gain is set to an inverting amplifier for exciting the crystal oscillator.
A crystal oscillation circuit for causing oscillation using a crystal oscillator comprises an inverting amplifier, a buffer, and a voltage shift circuit. The voltage shift circuit operates in such a way that within prescribed limits by which the output of the inverting amplifier satisfies excitation conditions of the crystal oscillator and by which the oscillation output of the buffer satisfies input conditions of a following circuit that follows the buffer, a supply voltage (Vdd) is reduced by a first voltage value, and a ground potential (GND) is increased by a second voltage value with respect to both the inverting amplifier and the buffer. That is, the voltage shift circuit comprises an n-channel metal-oxide semiconductor transistor whose gate threshold voltage matches the first voltage value, and a p-channel metal-oxide semiconductor transistor whose gate threshold value matches the second voltage value.
Specifically, the voltage shift circuit comprises an n-channel metal-oxide semiconductor field-effect transistor whose gate is supplied with a first bias voltage (BIAS1) for regulating upper limits of the excitation level of the crystal oscillator through a current path that is interposed in relation to the line of the supply voltage (Vdd), and a p-channel metal-oxide semiconductor field-effect transistor whose gate is supplied with a second bias voltage (BIAS2) for regulating lower limits of the excitation level of the crystal oscillator through a current path that is interposed in relation to the ground potential (GND). In addition, the voltage shift circuit can be subjected to reconfiguration in which the supply voltage is supplied to the gate of the n-channel metal-oxide semiconductor field-effect transistor, and the gate of the p-channel metal-oxide semiconductor field-effect transistor is grounded.
Thus, even though the gain of the inverting amplifier for exciting the crystal oscillator is increased to be relatively high, it is possible to prevent the crystal oscillator from being damaged while suppressing the excitation level of the crystal oscillator.